Hi Martin. I normally pick up off the treads of the wheel with just a straight piece of wire. This is something I have applied with success to models in various scales for some decades. I came across an idea from a UK manufacturer (of some standing) suggesting the pickup be a small solder filled loop.Whether it is the solder helping to contaminate the tread or the larger surface area better able to collect rubbish I have not bothered to investigate. Pickup was dodgy from day one. I gave it a fair trail but then went back to the straight piece of wire and now have a reliable loco as usual.